1. Field of the Invention
The invention relates to a filtering aid for the treatment of suspensions and particularly of domestic, industrial, and other sludges, for subsequent draining.
2. Description of the Prior Art
The draining of sludges, particularly of sewage sludges, is only possible with so-called filtering aids because of the small particle size of the sludge particles, the surface tension of the sludge liquor, the viscosity, the high compressibility, and other properties which lead to a high specific filter resistance of the sludges. Known filtering aids are iron salts and aluminum salts in connection with calcium hydroxides and, more recently, high molecular weight, water-soluble polymers with cationic character. By adding such filtering aids in a dosed manner and mixing them with the sludge before it is drained, due to the exchange of electric charges to the suspended sludge particles, the surfaces of the particles are destabilized and rendered capable of coagulation and flocculation. See U. Moller "Zur Technologie der Schlammentwasserung" (Concerning the Technology of Sludge Draining) Stadtehygiene 3/1967, pages 54 to 59.
The flakes or flocks formed by the use of polymers are, due to their structure, much looser than those obtained from a conventional coagulation so that the final water content of the mechanically drained sludge is smaller. However, they are very sensitive to mechanical stress, so that not all known draining methods can be readily used. Also, aeration of the sludge over a longer period also has a negative effect on the flakes and leads to their destruction. Since domestic sludges accumulate daily in large amounts, such filtering aids significantly increase the operating cost of sewage treatment plants. Finally, such chemical additives are a burden on the biological system, particularly when the accumulated sewage sludge is to be returned into the biological cycle in the form of compost.